Public health minister Anna Soubry is worried that poor people are fat these days. When she was a child they were taunted for being "skinny runts" you could spot a mile off, but now she reckons to be able to identify them not by how thin and hungry they are, but by their obesity.

She has a point. Class determines how obese you are, how long you live, what sort of illnesses you are likely to suffer from. But if Soubry wants to use her role to do something about it, she might like to investigate the current burger scandal for a less judgmental view of why these health inequalities persist.

Poor people eat a lot more cheap burgers and processed meats than more affluent households, who tend to choose more virtuous wholemeal bread and fruit and vegetables. This has more than a little to do with prices. A posh beefburger that is made of 90-100% beef will set you back about £1. An "economy" beefburger, on the other hand – the sort caught with horse and pig DNA by the Food Safety Authority of Ireland last week – could be bought for about 12p.

Some supermarkets have these economy beefburgers on special offer. This "special offer economy burger" may look just like the economy burger that was on sale at a higher price the week before – its packaging, for one, will be the same. But that's precisely the trouble with junk food: it rarely owns up to its secrets.

In fact, this "special offer economy burger" will probably have been made to a different specification under special contract to the supermarket. It will have been constructed down to a price fixed by the retail buyer. Many low-income families rely on these special offers, especially in recessionary times. If the burgers are down to 10p each, they look like a bargain. So what's in them? I asked a key player in the business of supermarket meat to explain how the process might work. Unsurprisingly, he did not want to be identified.

Economy beefburgers tend to have a meat content hovering around 60-65%, but by law a product with that description need be only 47% cow, as opposed to horse or pig or whatever bargain-basement ingredient a rogue supplier happens to have on hand. So our knock-down burger tends to have more fat and more water in its specification (water being the cheapest ingredient of all) and nicely heavy with it. It will also have additives to make sure the fat and water stay in when you cook it. The meat itself, that 47% legal minimum bit, isn't just what you and I think of as meat – that is, lean muscle meat, but is allowed to contain fat, collagen and connective tissue in the same proportion as they naturally occur in the cut being used.

So if a manufacturer wants to make a really cheap beefburger – he can't afford to run at a loss for too long, and even the powerful supermarket buyers recognise this – he can use a cut such as beef brisket. That's on average 32% fat, 19% connective tissue, and 3% collagen. And yes, collagen is what you are thinking, the stuff film stars inject into their lips to make them look bigger than they really are, and it's a top tool in the cheap meat industry's box. So the "meat", which is less than half the burger already, can in fact be a third fat.

During a promotion a manufacturer may be told he can add extra fat, maybe around 20% extra, which is all legal, so long as it's declared as added fat on the label. Beef fat on average contains 6% collagen. So you can whack in a bit more collagen as well as fat. The crucial thing to understand is that economy foods at rock-bottom prices such as these are a reconstruction of deconstructed parts, bought around the world from wherever is cheapest. Exchange rate fluctuations might affect where you want to buy your components from week to week. When our food is denatured and deracinated in these extenuated supply chains, perhaps we should not be surprised that the species sometimes get mixed up.

When a meat processor is constructing this product to hit a particular price, he and his supermarket buyer also know they have to hit the percentages laid down in the regulations. The knock-down burger, with its payload of fat and water, might tend to be a little light on protein, so the manufacturer might add concentrated proteins in various forms. These might be extracted from boiled-up bits of animal even the French wouldn't eat, such as hide or gristle and other offcuts, which are then dried and ground down to a powder that goes into a "seasoning" mix. They might alternatively be extracted from animal waste by chemical hydrolysis.

Burger jokes have been ten a penny over the past week, but the impact that this sort of junk has on people's lives is not funny at all. People on low incomes suffer far higher rates of diet-related disease, and not just obesity. They have higher rates of anaemia caused by lack of iron, especially in pregnancy.

Mothers from low-income groups are more likely to have children of low birthweight, who, in turn, are likely to suffer poor health and educational prospects as a result. They have more childhood eczema and asthma. They have higher rates of raised blood pressure, thanks to their processed diets. They are more likely to suffer diabetes, heart disease, vascular disease and strokes. They suffer more cancers of the lung, stomach and oesophagus. They have more cataracts caused by poor nutrition than those in other classes.

Adulteration of food, legal or otherwise, is no laughing or sneering matter.